The synthetic production of fuel gases, particularly methane, is an especially desirable manner of fulfilling world energy requirements. Fuel gases, particularly methane, can be transported long distances with little energy penalty, can be burned cleanly, and are extremely versatile. It has been proposed that a particularly advantageous manner of producing methane is the biogasification of peat. Peat is a very abundant resource in many parts of the world, and the production of methane from peat could provide a long-term supply of energy. For instance, in the United States the peat resources (in terms of energy content) are greater than the maximum recoverable energy from uranium, shale oil, or the combined reserves of petroleum and natural gas.
Conventional proposals for the production of methane from peat include conventional gasification and biogasification. The usual practice in conventional treatment is to gasify the peat with steam to produce CO and H.sub.2. The CO and H.sub.2 are then catalyzed to CH.sub.4. The high moisture content (80% after pressing) of the peat produces a severe energy penalty when proceeding in this manner, since the peat must be thermally dried either outside or inside the gasifier. Prior proposals for biogasification of peat suggest an alkaline oxidation pretreatment, separation of unreacted solids, and anaerobic fermentation of the liquid containing the soluble organics. Such prior biogasification proposals are usually capable of converting to methane only about 26% of the energy value in the peat, although unreacted peat solids may be further processed as a boiler fuel.
According to the present invention there is provided a method facilitating the efficient biogasification of peat, and particular apparatus and specific method steps therefor. The present invention has advantages over prior conventional gasification and biogasification methods in that it is possible to convert over 80% of the original energy of the peat to methane, and in addition produce some by-product energy.
According to a preferred method of the present invention, the first step in the production of fuel gas (primarily methane) from peat is the slurrying of the peat, such as to a consistency of about 3 to 5%. The slurried peat is then formed to a consistency of about 10 to 25%, such as by passing it through a cylinder mould. The peak slurry then is thoroughly mixed with alkali, such as by adding NaOH to the slurry in a repulper. The peat slurry is then heated to the solubilization temperature for phenolic polymers therein while it is pressurized, preferably by intimately mixing steam with the pulp slurry in a steam mixer.
Once the peat is at solubilization temperature, it is maintained at solubilization temperature and pressure conditions for a sufficient period of time so that substantially all of the peat becomes solubilized; this is accomplished continuously by feeding the heated slurry into a vertical vessel with a plurality of circular segment compartments which are sequentially filled and emptied. Once solubilized, the peat (after unsoluble components are separated therefrom) is fed to an oxidizer where oxidation takes place. This is preferably accomplished by passing the solubilized peat in a substantially vertical wavy path while introducing oxygen at the bottoms of segments of the path and withdrawing carbon dioxide from the tops of segments of the path. The exothermic heat of reaction is removed during oxidation, and the heated water generated in this heat removal is flashed to steam to supply steam to the steam mixer, and to provide by-product energy.
After oxidation, the solubilized peat is cooled to a temperature appropriate for fermentation (e.g. about 130.degree. to 150.degree. F.), and is then fermented aerobically. A gas including CO.sub.2 and fuel gas is produced by the fermentation process, and the CO.sub.2 is scrubbed from the gas produced by fermentation to provide the final product fuel gas (methane).
One component piece of apparatus for practicing the invention that is particularly important is the solubilizer. The solubilizer includes a vertically oriented pressure vessel that is circular in cross-section. Means are provided for dividing the vessel into a plurality of regular, uninterrupted vertically elongated compartments with each compartment extending the majority of the height of the vessel and the compartments are disposed in a closed path around the circumference of the vessel (each compartment comprising substantially a circular sector in cross-section). Means are provided for progressively feeding slurried peat or the like into each of the plurality of compartments, in turn, at the top thereof, and means are provided for progressively effecting emptying of each of the plurality of compartments at the bottom thereof after a predetermined retention time within the compartment, so that as one compartment is being filled another is being emptied. Progressive feeding and emptying is preferably accomplished by a common powered shaft disposed centrally in the vessel and extending the height thereof. The shaft operates one structure at the bottoms of the compartments to effect sequential discharge from the compartments, while at the top the shaft is provided with a chute which trails the bottom-emptying structure in the direction of rotation of the shaft so that as a compartment is emptied the next compartment is being filled.
Another important component piece of apparatus comprises the oxidizer. The oxidizer includes means defining a liquid-transporting substantially vertical wave path including an inlet and outlet. Means are provided for introducing oxygen at the bottoms of segments of the wave path, and means are provided for exhausting off-gases from the tops of segments of the wave path so that the wave path is maintained above atmospheric pressure. Means are provided for circulating coolant (particularly water) around the wave path to remove the exothermic heat of reaction from solubilized peat or the like in the wave path, while preventing mixing of the coolant and solubilized peat or the like circulating in the wave path. The wave path is preferably defined by a vessel having a plurality of vertically extending baffles disposed therein, the baffles alternately extending from the top and the bottom of the vessel each to define liquid passageway past the bottom or top thereof, respectively. The coolant circulation means preferably comprise a plurality of staggered heat-exchanger tubes extending in substantially horizontal planes within the vessel.
In addition to comprising a complete method for production of methane from peat and particular apparatus components adapted to practice the method, the invention also contemplates particular substeps of the general methane production process.
It is the primary object of the present invention to provide an energy efficient process, and apparatus adapted to be utilized therein, for the biogasification of peat. This and other objects of the invention will become clear from an inspection of the detailed description of the invention and from the appended claims.